Check all the solder connections. (It's easy to miss one)
Make sure there are no bent leads on any of the ICs.
Make sure the card's edge connector is clean, solder flux is hard to see. Use an eraser and very gently clean the edge connector if it looks suspect.
The same is true for the IC sockets, solder flux will prevent a good connection with the IC. I use a 90+% solution of Isopropyl Alcohol, you can buy it in any drug store. The ICs need to be out of the sockets. I completely submerse the board in alcohol and scrub the solder flux residue off with a stiff brush then let the board air dry. Do this in a well ventillated area away from any open flames. (duh)

On a IIGS the button is not enabled until the DDT card has been initialized. I've got a program that initialized the DDT card automatically on boot up but I think I forgot to put it on my web site. I'll check. Note! On the IIGS the DDT must be in slots 4,5,6 or 7.

The button generates an NMI and should only be used to interrupt a running program. A more controlled way of debugging progams is to use breakpoints.

Hi, all. Finally got around to building a DDT16 from the kit. Unfortunately, it is not operational. No smoke or flames, but entering the monitor and typing, e.g. C700G, just hangs. I've tried a couple of different slots with no better luck. The EPROM verifies fine against the binary and I've double and triple checked for bad or missing solder connections, misaligned chip pins, etc.

Where do I start trying to figure this out? From the Apple side, the slot ROM appears in the correct place and data looks correct. I'm guessing that it's going south when the CPU bank register is hit. What part of the hardware deals with this? Any tips appreciated!